Fix -Wno-sometimes-uninitialized by initializing "result" in parse_sax

The function parse_sax() returns sometimes-uninitialized warnings in
some compilers. This commit fixes that.

🐛 fixed callback-related issue (https://github.com/nlohmann/json/issues/971#issuecomment-413678360)

Refactor/no virtual sax

[RFC] Introduce a new macro function: JSON_INTERNAL_CATCH

Fix unit tests that were silently skipped or crashed (depending on the compiler)

These tests never worked - they weren't run before
d5aaeb4c.

Note that these tests would fail because of this library
ordering dictionary keys (which is legal). So changing the
input files (or modifying stored cbor/msgpack/ubjson files)
would make the tests work and they could get removed from
"exclude_packaged".

Also move parsing of files in these unit tests to within
the inner sections, so that they're only parsed
number_of_files * number_of_sections instead of
number_of_files * number_of_files * number_of_sections
(so, instead of close to 100k parses about 700).

sections, see also https://github.com/catchorg/Catch2/issues/816#issuecomment-278268122

As a result, when built with gcc, loop iterations were skipped. When
built with clang, the test aborted with an assertion in catch.hpp
line 6222.

This also addresses the issues discussed here:
https://github.com/nlohmann/json/issues/1032#issuecomment-378707696

and here:
https://github.com/catchorg/Catch2/issues/1241

Please note that this introduces new problems, as some of
the unit tests fail now - the library stores keys in
lexographical order, while the cbor/msgpack/ubjson examples
store them in original order.

Error in AppVeyor:

unit-conversions.cpp(441): error C2440: 'initializing': cannot convert from 'nlohmann::json' to 'std::nullptr_t'

(https://ci.appveyor.com/project/nlohmann/json/build/2838/job/wo7im01sq6tvhe9m)

Fixed compiler error in VS 2015 for debug mode

https://github.com/nlohmann/json/issues/1114

fixed compile error for #1045